What type of threads (cords) are used for the garlands to worship the Gods ?

The thread is known as Sutram or Tantuh in Sanskrit language. The thread (Sutram) is the basis of the flowers or beads to become as Garland.

In the Hindu Vedic Scripts there is one most distinguished name called Prasthanatrayam to study, to analyze and to practice for the self realization. The Prasthanatrayam contains three scripts which were derived from the Vedas. They are respectivesly. Brahma Sutras, the Upanishads and the Bhagavdgita. All of them ultimately preaches Athma Tatwam which is the Self Knowledge. Among these the first one Brahma Sutras. Ofcourse, there are another Sutras called Narada Bhakti Sutras and Sandilya Bhakti Sutras relate to the Devotion to the God. In a Garland the flowers are studded like wise the Aphorisms are studded.

Coming to the point, for a Garland the flowers and the thread are needed. The colours and fragrant of flowers may be different, but the thread should be strong for the basis of the flowers. The Garlands are made for the decorating the Gods to express our great devotion in different forms. But the same kind of thread should not be used for all the Gods.

For worship the Lord Sri Vishnu, if the Garland is used, the thread should be made of silk or yarn.

To worship the Goddess Parvati , in the Garland, the thread should be made of Silk. For worshipping Lord Siva, the thread in the Garland should be of wollen. The Sun God who is the prime source for all the creatures for giving the food, water etc., must be worshipped with the Garland made of silk or yarn thread.
The devotee who does the worship to the respective God or Goddess, he also wear the cloths made of that type of thread ( either silk, yarn etc .,).

It is said if it is happend to wear the Garland , then he should make the garland for himself and wear it which is superior. If not, if others make the garland for us then the Garland should be purified by worshipping it and then wear it. These kinds of conditions are said even for the customs and rituals